The ultrastructure of canine-secretory component and J-chain are being investigated now that they have been isolated and purified from secretory IgA; the combination of these chains with 7S IgA monomer may elucidate the assembly of the secretory IgA molecule. While investigating the origin of precursor cells that differentiate into immunoglobulin secreting plasma lymphocyte cells in the gastrointestinal tract, the interactions of orally administered antigens and processing cells (T-lymphocytes) in gut-associated lymphoid tissue (Peyer's patches) were studied in adult guinea pigs. A detailed identification of lymphoid cell populations in Peyer's patches and an assessment of lymphocyte activity (afferent and efferent T-cell functions) were made. To further assess primary immune responses in the gastrointestine, a rat model of Trichinella spiralis gut infection was used to observe the sequence of developing cellular immunity in Peyer's patches and other lymphoid tissues. In this parasitic gut infection, the Peyer's patch lymphoid tissue did not appear to be the first site of cellular responsiveness but rather to acquire cellular reactivity only when other lymphoid elements in the infected host have also required similar antigen-induced reactivity.